This invention relates to compositions comprising polymeric binder and optional reflective elements for use in marking trafficable surfaces.
Various striping or marking compositions have been used on roads, highways, parking lots, and recreational trails for many years. Such compositions are typically used to form stripes, bars, and other markings for the delineation of lanes, crosswalks, parking spaces, symbols, legends, and the like. The compositions are often applied by spray coating (i.e., painting) a pavement surface.
Conventional liquid pavement marking compositions include alkyd-based compositions and epoxy-based compositions. Such compositions typically dry slowly and contain chemical linkages (e.g., ester linkages) that are susceptible to degradation (e.g., by hydrolysis), leading to poor weatherability upon application to pavement surfaces. The compositions also typically require the use of organic solvents or drying oils (in order to obtain sprayable viscosities) or the use of drying accelerators, which can lead to discoloration.
In addition to alkyd- and epoxy-based compositions, isocyanates plus polyethers or polyamines have been utilized as two-part compositions to provide polyurethane- or polyurea-based pavement markings. Such compositions have provided improvements in drying times, weatherability, and resistance to discoloration (relative to alkyd- and epoxy-based compositions), but require the handling of moisture sensitive, potentially hazardous isocyanates.
Numerous other pavement marking compositions have been developed to get faster drying times by using heated application of materials to road surfaces. Also, waterborne coatings based on acrylic lattices and epoxy resin emulsions have been suggested for use in pavement marking. Such compositions have the advantages of low volatile organic content and one-part coating application but are not yet sufficiently durable for long term road presence and retained reflectivity performance. The former also require the use of expensive heating equipment.
Acrylate-based coating compositions have provided a balance of stability and fast cure (even at low temperatures) but have required the use of low molecular weight, ethylenically unsaturated compounds that have significant vapor pressures and are thus somewhat difficult to handle at higher ambient temperatures. Preformed pavement marking sheets or tapes have been used for their ease of handling but often exhibit adhesion problems, especially at road intersections.
Thus, there exists a continuing need in the art for pavement marking compositions that can be easily and safely handled and applied without the need for expensive equipment, that can be quickly cured even at ambient temperatures, and that are durable under a wide range of weather and use conditions.
Briefly, in one aspect, this invention provides a pavement marking composition comprising (a) a polyfunctional ethylenically unsaturated polymer selected from the group consisting of polyfunctional ethylenically unsaturated polyurethanes, polyureas, polythiocarbamates, polythiocarbamateurethanes, polythiocarbamateureas, and polyurethaneureas comprising at least one chain extender-derived segment and at least one polycarbonate, polyether, or polyester segment; and (b) at least one ethylenically unsaturated monomer. (As used herein, the term xe2x80x9cchain extenderxe2x80x9d refers to a polyfunctional, molecular weight increasing co-reactant.) Preferably, the polyfunctional polymer comprises at least one polycarbonate or polyester segment, and the ethylenically unsaturated monomer is a (meth)acryloyl-functional monomer having a molecular weight greater than about 150. Most preferably, the composition further comprises a curing system, filler, pigment, and/or reflective elements.
Relative to known liquid pavement marking compositions, the pavement marking composition of the invention provides durably bondable pavement markings that surprisingly exhibit both improved cold impact (snow plow) resistance and improved wear resistance, even though these characteristics are generally difficult to simultaneously achieve and/or enhance. The composition can be easily applied (e.g., by hand using a trowel or a drawbox or by spraying), without the need for expensive and/or bulky heating equipment, and cures in a reasonable amount of time at any of a wide range of commonly-encountered temperatures. Furthermore, since the composition does not contain either solvent or reactive isocyanate (nor, in preferred embodiments, low molecular weight monomer), it can be safely handled with reduced inhalation risk and environmental hazard.
In other aspects, this invention also provides a pavement marking comprising the cured pavement marking composition of the invention, a trafficable surface bearing the pavement marking, and a process for marking or for applying a pavement marking.
Polyfunctional ethylenically unsaturated polymers suitable for use in the composition of the invention are those polyfunctional ethylenically unsaturated polyurethanes, polyureas, polythiocarbamates, polythiocarbamateurethanes, polythiocarbamateureas, and polyurethaneureas that comprise at least one chain extender-derived segment and at least one polycarbonate, polyether, or polyester segment. The polymers can be prepared by the reaction of at least one polyisocyanate; at least one hydroxyl-, thio-, or secondary amino-polyfunctional polycarbonate, polyether, or polyester; at least one isocyanate-reactive polyfunctional chain extender; and at least one ethylenically unsaturated monomer having additional functionality, e.g., hydroxyl or amine, that is reactive with isocyanate.
Useful polyisocyanates include aliphatic, alicyclic, and aromatic diisocyanates, and mixtures thereof. A number of such diisocyanates are commercially available. Representative examples of suitable diisocyanates include hexamethylene diisocyanate (HDI), trimethyl hexamethylene diisocyanate (TMHDI), m- and p-tetramethylxylene diisocyanate (TMXDI), diphenylmethane diisocyanate (MDI), napthalene diisocyanate (NDI), phenylene diisocyanate, isophorone diisocyanate (IPDI), toluene diisocyanate (TDI), bis(4-isocyanatocyclohexyl)methane (H12MDI), and the like, and mixtures thereof. Useful polyisocyanates also include derivatives of the above-listed monomeric polyisocyanates. These derivatives include, but are not limited to, polyisocyanates containing biuret groups, such as the biuret adduct of hexamethylene diisocyanate (HDI) available from Bayer Corp., Pittsburgh, Pa. under the trade designation Desmodur(trademark) N-100, polyisocyanates containing isocyanurate groups, such as that available from Bayer Corp., Pittsburgh, Pa. under the trade designation Desmodur(trademark) N-3300, as well as polyisocyanates containing urethane groups, uretdione groups, carbodiimide groups, allophonate groups, and the like. If desired, small amounts of one or more polyisocyanates having three or more isocyanate groups can be added to effect a degree of crosslinking. Preferred polyisocyanates include aliphatic diisocyanates and derivatives thereof, with IPDI being most preferred.
Useful polycarbonates, polyethers, and polyesters include those which have an equivalent weight in the range of about 250 to about 5000 (preferably from about 400 to about 2,500, more preferably from about 500 to about 1,500) and which have an average secondary amino-, thio-, and/or hydroxyl-functionality of about 2.0 to about 3.0 (preferably of about 2.0 to about 2.8, more preferably of about 2.0 to about 2.5). A number of such functional polymers are commercially available. Diols are preferred due to their availability, low cost, and stability. Representative examples of polymers that are useful (when functionalized in the foregoing manner) include aliphatic polycarbonates such as polyestercarbonates and polyethercarbonates; polyethers such as polyethylene glycol, polypropylene glycol, polybutylene glycol, and polytetrahydrofuran; polyesters such as polycaprolactones, polybutylene adipate, polydiethylene adipate, poly(3-methyl-1,5-pentane)adipate, and poly(neopentyl/1,6-hexane)adipate; and mixtures thereof. Polycarbonates and polyesters are preferred because of their excellent UV-, heat-, and oxidation stability, with polycarbonates being most preferred because of their excellent hydrolysis resistance.
Useful chain extenders include low molecular weight (e.g., below about 1000, preferably below about 600) polyols, e.g., ethylene glycol, 1,2- and 1,3-propanediol, 1,3- and 1,4- and 2,3-butanediol, diethylene glycol, dipropylene glycol, tripropylene glycol, 1,6-hexanediol, 1,4-cyclohexane dimethanol, 3-methyl 1,5-pentanediol, and neopentyl glycol; low molecular weight secondary polyamines, e.g., 1,3-di(4-piperidyl)propane (DIPIP), N(2-aminoethyl propylmethyldimethoxysilane (DAS), piperazine, N,Nxe2x80x2-dialkyl(methylene)dianiline, N,Nxe2x80x2-dialkyl(1,4-diamino)benzene, N,Nxe2x80x2-bis(diisopropylmethyl)diaminoethane, and N,Nxe2x80x2-bis(t-butyl)diamino cyclohexane; and the like; and mixtures thereof. Although difunctional chain extenders are generally preferred, small amounts of one or more chain extenders having three or more isocyanate-reactive functional groups can be added, if desired.
Preferred chain extenders are the secondary amine reaction products of maleic acid esters and polyamines, i.e., aspartic ester polyamines. Such chain extenders are preferred, e.g., for the polymer properties that they provide. Preferred aspartic ester polyamines have the following formula 
wherein R1 is a divalent organic group (preferably having from 1 to about 40 carbon atoms) and each R is independently an organic group inert toward isocyanate groups at temperatures of 100xc2x0 C. or less.
In the above formula, preferably R1 is an aliphatic group (preferably having from 1 to about 40 carbon atoms), which can be branched, unbranched, or cyclic, and more preferably, R1 is selected from the group of divalent hydrocarbon groups obtained by the removal of the amine groups from 1,4-diaminobutane, 1,6-diaminohexane, 2,2,4- and 2,4,4-trimethyl-1,6-diaminohexane, 1-amino-3,3,5-trimethyl-5-aminomethyl-cyclohexane, 4,4xe2x80x2-diamino-dicyclohexyl methane, or 3,3-dimethyl-4,4xe2x80x2-diamino-dicyclohexyl methane. Divalent hydrocarbon groups obtained by the removal of the amine groups from relatively high molecular weight polyether polyamines containing aliphatically-bound primary amine groups, such as the products marketed under the trade designations Jeffamine(trademark) (by Texaco and Huntsman) and Hycar(trademark) (by B.F. Goodrich), are also suitable. Other suitable polyamine precursors include ethylene diamine, 1,2-diaminopropane, 1,6-diaminohexane, 2,5-diamino-2,5-dimethylhexane, 1,11-diaminoundecane, 1,12-diaminododecane, 2,4- and/or 2,6-hexahydrotoluylene diamine, and 2,4xe2x80x2-diamino-dicyclohexyl methane. Aromatic polyamines such as 2,4- and/or 2,6-diaminotoluene and 2,4xe2x80x2- and/or 4,4xe2x80x2-diaminodiphenyl methane are also suitable but less preferred, due to their lower reactivity and tendency to yellow.
In the above formula, preferably, each R is independently an organic group having from 1 to about 40 carbon atoms, more preferably each R is independently an alkyl group (preferably having from 1 to about 20 carbons), which may be branched or unbranched, and most preferably each R is independently a lower alkyl group (having from 1 to about 4 carbon atoms).
Suitable aspartic ester polyamines are commercially available from Bayer (Pittsburgh, Pa., U.S.A. or Leverkusen, Germany) under the trade designations Desmophen(trademark) XP-7053, XP-7059, VP-LS 2973, and XP-7109, for example. Desmophen(trademark) XP-7053 is substantially composed of the following compound, N,Nxe2x80x2-(4,4xe2x80x2-dicyclohexylmethane-diyl)bis tetraethyl aspartic ester: 
Desmophen(trademark) XP-7059 is substantially composed of the following compound, N,Nxe2x80x2-(hexane-1,6-diyl)bis tetraethyl aspartic ester: 
Desmophen(trademark) XP-7109 is substantially composed of the following compound, N,Nxe2x80x2-(2-methylpentane-1,5-diyl)tetrabutyl aspartic ester: 
Desmophen(trademark) VP-LS 2973 (preferred) is substantially composed of the following compound, N,Nxe2x80x2-(3,3-dimethyldicyclohexylmethane-4,4xe2x80x2-diyl)-bis-tetraethyl aspartic ester: 
Other suitable aspartic ester polyamines include, e.g., N,Nxe2x80x2-(bis-2-propyl)polypropyleneglycol 300-O,Oxe2x80x2-diyl-bis-tetraethyl aspartic ester, N,Nxe2x80x2-(butyl-1,4-diyl)-bis-tetraethyl aspartic ester, N,Nxe2x80x2-(2,2-dimethylpropane-1,3-diyl)-bis-tetraethyl aspartic ester, N,Nxe2x80x2-(2,4-dimethylhexane-1,6-diyl)-bis-tetraethyl aspartic ester, and the like, and mixtures thereof.
Suitable ethylenically unsaturated monomers for use in preparing the polyfunctional polymer are those which further comprise isocyanate-reactive functionality, e.g., hydroxyl or amine functionality. Representative examples of suitable monomers include 2-hydroxyethyl methacrylate, 3-hydroxypropyl methacrylate, 2-hydroxyethyl acrylate, 3-hydroxypropyl acrylate, 4-hydroxybutyl acrylate, 4-hydroxybutyl methacrylate, allyl alcohol, allylethyl alcohol, oleyl alcohol, 4-vinylbenzyl alcohol, and the like, and mixtures thereof. Preferred ethylenically unsaturated monomers are (meth)acryloyl functional. Most preferred is 2-hydroxyethyl methacrylate.
The polyfunctional polymer can be prepared by first combining at least one hydroxyl-, thio-, or secondary amino-polyfunctional polycarbonate, polyether, or polyester (about 0.1 to about 2.5 moles) with at least one isocyanate (about 2.0 to about 3.0 moles), optionally in the presence of a solvent. If desired, the ethylenically unsaturated monomer described infra can be present at this stage to function as a solvent. The resulting mixture can be allowed to react for about 1 hour at about 40-60xc2x0 C. under a dry, inert gas atmosphere, generally with stirring. About 0.01% of an organometallic catalyst, e.g., of tin or bismuth, can be utilized, as further explained below. Chain extender(s) (about 0.1 to about 1.5 moles) can then be added to the mixture. However, the chain extender can be part of the initial mixture (of polymer and isocyanate) described above, if desired. The reaction can be allowed to continue for a period of about 3 hours until the isocyanate content is near the theoretical value. Finally, the isocyanate-reactive, ethylenically unsaturated monomer(s) can be added (about 0.2 to about 5.6 moles) and reaction continued for a period of about 2 hours to provide polyfunctional polymer.
Preferably, a catalyst is used in preparing the polyfunctional polymer. Catalysts for reacting isocyanates and active hydrogen-containing compounds are well known in the art. Preferred catalysts include organometallic compounds and amines. Useful organometallic compounds include organotin compounds such as dimethyltin dilaurate, dibutyltin dilaurate, dibutyltin dimercaptide, bis lauryl(dibutyltin)oxide, and the like, and mixtures thereof. Zinc or bismuth compounds are also useful. Amine catalysts include tertiary amines, such as, for example, diazobicyclooctane. A preferred catalyst is dibutyltin dilaurate. Catalyst is used in an amount effective to provide a desired reaction rate. Preferably, catalyst is used in an amount of about 0.01-2 percent by weight (more preferably, 0.01-0.03 percent by weight), based on the total weight of solids.
Water can accelerate the reaction of aspartic ester amines with isocyanates. Free water may be present in the system or adsorbed on the polyfunctional polycarbonate, polyether, or polyester reactant or on any added pigments or fillers.
The polyfunctional polymer can be used to prepare the pavement marking composition of the invention by combining the polymer with reactive diluent, i.e., with at least one ethylenically unsaturated monomer. Suitable monomers are those which are capable of reacting with the polyfunctional polymer. Preferred monomers are (meth)acryloyl-functional monomers, more preferably (meth)acryloyl-functional monomers having a molecular weight of at least about 150 and/or a vapor pressure of less than about 43 mbar at 20xc2x0 C. (most preferably less than about 10 mbar at 20xc2x0 C.)
Representative examples of suitable monomers include methyl acrylate, ethyl acrylate, n-butyl acrylate, isobutyl acrylate, t-butyl acrylate, ethyl methacrylate, butyl methacrylate, ethyltriglycol methacrylate, isobornyl acrylate, 2-(((butylamino)carbonyl)oxy)ethyl acrylate, acetoacetoxyethyl methacrylate, acetoacetoxyethyl acrylate, acetoacetoxypropyl acrylate, acetoacetoxybutyl acrylate, 2-methyl-2-(3-oxo-butyrylamino)-propyl methacrylate, 2-ethylhexyl acrylate, n-octyl acrylic acetate, decyl acrylate, lauryl acrylate, stearyl acrylate, 2-hydroxyethyl acrylate, 2-hydroxypropyl acrylate, 2-hydroxyethyl methacrylate, 2-hydroxypropyl methacrylate, xcex2-ethoxyethyl acrylate, 2-cyanoethyl acrylate, cyclohexyl acrylate, diethyl aminoethyl acrylate, hexyl methacrylate, decyl methacrylate, lauryl methacrylate, stearyl methacrylate, phenylcarbitol acrylate, nonylphenyl carbitol acrylate, nonylphenoxy propyl acrylate, N-vinyl pyrrolidone, polycaprolactam acrylate, acryloyloxyethyl phthalate, acryloyloxy succinate, 2-ethylhexyl carbitol acrylate, xcfx89-carboxy-polycaprolactam monoacrylate, phthalic acid monohydroxyethyl acrylate, styrene, vinyl acetate, vinyl toluene, xcex1-methyl styrene, acrylonitrile, gycidyl methacrylate, n-methylol acrylamide-butyl ether, n-methylol acrylamide, acrylamide, dicyclopentenyloxyethyl acrylate, dicyclopentenyl acrylate, dicyclopentenyloxyethyl acrylate, and the like, and mixtures thereof.
Preferred monomers include ethyl acrylate, n-butyl acrylate, isobutyl acrylate, t-butyl acrylate, methyl methacrylate, ethyl methacrylate, butyl methacrylate, ethyltriglycol methacrylate, isobornyl acrylate, 2-(((butylamino)carbonyl)oxy)ethyl acrylate, acetoacetoxyethyl methacrylate, acetoacetoxyethyl acrylate, acetoacetoxypropyl acrylate, acetoacetoxybutyl acrylate, 2-methyl-2-(3-oxo-butyrylamino)-propyl methacrylate, 2-ethylhexyl acrylate, and mixtures thereof, with isobornyl acrylate, 2-(((butylamino)carbonyl)oxy)ethyl acrylate, ethyltriglycol methacrylate, 2-ethylhexyl acrylate, 2-ethylhexyl methacrylate, acetoacetoxyethyl methacrylate, acetoacetoxyethyl acrylate, acetoacetoxypropyl acrylate, acetoacetoxybutyl acrylate, 2-methyl-2-(3-oxo-butyrylamino)-propyl methacrylate, and mixtures thereof being more preferred.
If desired, small amounts of multifunctional ethylenically unsaturated monomer(s) (compounds possessing at least two polymerizable double bonds in one molecule, e.g., multifunctional acrylates or methacrylates) can be utilized to, e.g., effect crosslinking. Representative examples of such multifunctional monomers include ethylene glycol diacrylate; 1,2-propylene glycol diacrylate; 1,3-butylene glycol diacrylate; 1,6-hexanediol diacrylate; noepentylglycol diacrylate; trimethylolpropane triacrylate; polyoxyalkylene glycol diacrylates such as dipropylene glycol diacrylate, triethylene glycol diacrylates, tetraethylene glycol diacrylates, polyethylene glycol diacrylate; ethylene glycol dimethacrylate; 1,2-propylene glycol dimethacrylate; 1,3-butylene glycol dimethacrylate; 1,6-hexanediol dimethacrylate; neopentylglycol dimethacrylate; bisphenol-A-dimethacrylate; diurethane dimethacrylate; trimethylolpropane trimethacrylate; polyoxyalkylene glycol dimethacrylates such as dipropylene glycol dimethacrylate, triethylene glycol dimethacrylates, tetraethylene glycol dimethacrylates, polyethylene glycol dimethacrylate; N,N-methylene-bis-methacrylamide; diallyl phthalate; triallyl phthalate; triallyl cyanurate; triallyl isocyanurate; allyl acrylate; allyl methacrylate; diallyl fumarate; diallyl isophthalate; diallyl tetrabromophthalate, and the like, and mixtures thereof.
In preparing the pavement marking composition of the invention, the polyfunctional polymer can be combined with at least one ethylenically unsaturated monomer in a ratio of about 1:10 to about 10:1 (preferably about 3:10 to about 1:1, more preferably about 2:3) and the resulting mixture allowed to react for a period of about 15 to about 30 minutes. Although the composition can be cured by exposure to ultraviolet (if the composition is only lightly filled) or electron beam radiation, thermal curing is generally preferred. If radiation curing is utilized, one or more photoinitiators, e.g., benzophenone can be added, if necessary or desired, e.g., in amounts ranging from about 0.05 to about 5 weight percent. Representative examples of suitable photoinitiators include 2,2-dimethoxy-1,2-diphenylethane-1-one, 1-hydroxy-cyclohexyl-phenylketone, 2-methyl-1-[4-(methylthio)phenyl]-2-morpholinopropanone-1,1-hydroxy-cyclohexyl-phenylketone, benzophenone, and the like, and mixtures thereof.
Preferably, however, a curing system comprising a thermal polymerization initiator and, preferably, an accelerator is utilized (e.g., in amounts of from about 0.01 to about 5 weight percent of each). Useful thermal polymerization initiators include organic peroxides, e.g., diacyl peroxides, dialkyl peroxides, hydroperoxides, ketone peroxides, and the like, and mixtures thereof. The accelerator of the curing system, if an accelerator is used, functions to decompose the initiator through a redox reaction and thereby facilitates the generation of active radicals. (Alternatively, heat and pressure can be utilized to accelerate reaction.) Useful accelerators include metal salts, e.g., cobalt naphthenate and vanadium octoate; tertiary amines; mercaptanes, e.g., dimethyl-p-toluidine, diisopropoxy-p-toluidine, diethyl-p-toluidine, dimethyl aniline, and glycol dimercaptoacetate; and the like; and mixtures thereof. The accelerator can be included in the initial pavement marking composition, with the initiator preferably being added just prior to application of the composition to a trafficable surface.
The pavement marking composition of the invention can further comprise other optional components, if desirable for a particular application or use. For example, weathering additives such as ultraviolet absorbers, hindered amine light stabilizers, antioxidants, dispersing and grinding aids, wetting agents, adhesion promoters, impact modifiers (e.g., rubber tougheners such as those available under the trade designations Paraloid(trademark) 2691 and EXL-2330 from Rohm and Haas), defoamers, suspension stabilizers, biocides, etc., can be added to improve the manufacturability and the overall durability of markings of the present invention.
Also, pigments (e.g., TiO2), fillers (e.g., talc, CaCO3, clay, and hollow glass microspheres), extenders, diluents, plasticizers, leveling agents, and surfactants can be used. Pigments are well known in the pavement marking art to impart desired visual appearance properties in the daytime and contribute to reflective performance of the marking at night. Fillers and extenders can be used to modify flow properties of the liquid coating and contribute to the bulk volume of the final coating with lower cost per volume materials. The pigments, fillers, and extenders can have a significant impact on uncured formulation and cured film density, film cure profile and track free time, cured film modulus, coating adhesion to a substrate, response to thermal cycling, possible shrinkage of polymer components, abrasion, and coating durability. Typically at least about 15 weight percent of the final dried coating is made up of such non-soluble material, more preferably, at least about 20 weight percent and, most preferably, at least about 50 weight percent. In some instances, about 30 weight percent to about 80 weight percent of pigments, fillers, and extenders can be used.
Preferably, the pavement marking composition of the invention includes drying wax to prevent oxygen inhibition of the radical curing. (Alternatively, an inert gas atmosphere can be provided.) Suitable waxes include paraffin waxes, polyethylene waxes, stearic acids, and the like, and mixtures thereof. Paraffin waxes are generally preferred. Drying wax can generally be used in an amount of about 0.2 to about 5 (preferably about 0.3 to about 2.5) parts per 100 parts of the composition.
The pavement marking composition of the invention can be applied directly to a trafficable surface with or without a primer layer or to a substrate that is applied to the surface. This can be easily done by hand using simple equipment, e.g., a trowel or a drawbox, or by spray coating techniques. Typically, a polyfunctional polymer/ethylenically unsaturated monomer/accelerator component and an initiator component can be applied using a spraying apparatus (e.g., equipment available from Hofmann as Model Numbers H 26-2 Universal and H 33-2 Universal) that allows the components to combine (e.g., in a ratio of about 98 parts of the polymer/monomer/accelerator mixture to about 2 parts initiator) and mix (either statically or dynamically) immediately prior to exiting the apparatus. Alternatively, a component comprising polymer, monomer, and accelerator can be combined immediately prior to exiting the apparatus with (i) a component comprising polymer, monomer, and initiator; or (ii) with both a component comprising polymer and monomer and a component comprising initiator. Other possible combinations will be recognized by those skilled in the art.
Although not necessary, more complex application equipment can be utilized, if desired. For example, two-component, high pressure, airless, impingement mixing systems can be used. Also, plural component spray equipment with a static mixer can be used. An example of an airless, impingement mixing spray system is manufactured by Gusmer (1 Gusmer Drive, Lakewood, N.J. 08701-0110). The system includes the following components: a proportioning section which meters the two components and increases the pressure to above about 1500 psi (103 bar); a heating section to raise the temperatures of the two components (preferably, independently) to control viscosity; and an impingement spray gun which combines the two components and allows mixing just prior to atomization. Other manufacturers of impingement systems include Binks Manufacturing (Chicago, Ill.) and Probler.
Another more complex system that can be used for applying the composition of the invention is a system which uses a static mix tube to achieve blending of the polymer/monomer and initiator components. The system is similar to that of the impingement unit in that it meters, builds pressure, and heats the components. However, at the spray gun, the components are combined and pumped through a length of tubing which contains flights designed to mix the components prior to atomizing. This system requires periodic flushing of the static mix tube to prevent accumulation of cured composition which could plug the spray gun. An example of such a spray gun is a Binks Model 43P.
The pavement marking composition of the invention cures rapidly at a wide range of application temperatures, e.g., at temperatures as low as about 0xc2x0 C. and as high as about 40xc2x0 C., to provide markings that exhibit excellent wear and impact resistance characteristics. The markings have conventional daytime visibility, but, if desired, reflective optical elements can be added to the pavement marking composition to enhance night time visibility. Generally, the reflective elements are no greater than about several millimeters in average diameter. When the reflective elements are ceramic, e.g., glass, microspheres, they are typically in the range of about 200 micrometers to about 600 micrometers in average diameter.
The reflective elements can be in the form of glass beads (also referred to as microspheres or microsphere lenses) that are preferably light transmissible. Chemical treatment of bead surfaces, such as with an organochromium compound, may be utilized as known in the art to enhance resin to glass adhesion. Other chemical coupling agents such as silanes, titanates, and zirconates are also known. Additionally, fluorocarbon treatment of the glass beads can aid in achieving substantially uniform hemispherical bead sinkage.
Ceramic, e.g., glass, beads or microspheres can also be used as a filler, in addition to providing night time reflectivity. The beads or microspheres may function similarly to mineral particulates on the wear surface of a coated abrasive (sandpaper), protecting the polymeric binder from applied stresses. The microsphere average diameter, application or coverage rate (weight per unit area), and surface chemistry (e.g., treatment with coupling agent) affect the durability of pavement markings. Preferably, coverage rates are greater than about 6 grams of beads per 4 inch lineal foot (10xc3x9730 cm area), for a 15 mil (0.4 mm) thick cured film, more preferably greater than about 15 grams, even more preferably, greater than about 38 grams and, most preferably, greater than about 45 grams of beads per 300 cm2 area for a 0.4 mm thick cured film.
The ceramic beads can be applied directly to the composition coated on a pavement surface. Alternatively, retroreflective optical elements having vertical surfaces can be utilized. Vertical surfaces provide better orientation for retroreflection. Also, vertical surfaces may prevent the build-up of a layer of water over the retroreflective surface during rainy weather, which otherwise interferes with the retroreflection mechanism.
One type of such retroreflective optical elements with vertical surfaces is made up of pellets comprising a thermoplastic core coated with ceramic beads. Such reflective elements are made by the introduction of pellets of a thermoplastic polymer to a heated bed of ceramic beads, either a fluidized bed or a rotary kiln. The temperatures and residence times are such that the beads adhere to the surface polymer pellets, become partially embedded in the surface, and prevent the pellets from agglomerating with one another. Polymers that can be used as the cores of thermoplastic reflective elements include ethylene methacrylic acid (EMAA) and ethylene acrylic acid (EAA) copolymers, polycarbonate, and thermoplastic polyurethanes. Such reflective elements provide controlled delivery of the beads. The elements and methods of making them are described in U.S. Pat. No. 5,750,191 (Hachey et al.), the descriptions of which are incorporated herein by reference.
Layered elements have been made using polymers having differing melt behavior. The retroreflective elements include a plurality of optical elements, e.g., ceramic beads, partially embedded in the vertical surfaces of the central layer of the reflective element""s core. The core has a central layer having a first and a second major surface and at least one vertical surface. On the first and the second major surfaces, a barrier layer is applied. The central and barrier layers"" compositions are selected such that a plurality of optical elements, when heated to the embedment temperature, embed and adhere to the central layer while the barrier layers are substantially free from optical elements.
One method of forming these retroreflective elements is by extruding a central layer between the barrier layers and calendering to the desired thickness. Suitable central layer material includes polymeric materials, both thermoplastic and thermoset, and mixtures thereof. This composite is then processed into cores of desired shape and size. The cores are then combined with the optical elements such that the optical elements embed and adhere in the vertical surface(s) of the central layer at the embedment temperature. Sandwich constructions can be made by laminating a lower melting material between two layers of higher melting polymer or coating a crosslinkable coating on the upper and lower surfaces of a sheet of lower melting polymer. The composite sheet can be cut into the desired shape and the shaped particles coated with optical elements such as beads or microspheres in a way similar to the thermoplastic elements to yield elements with beads selectively placed only on the sides (vertical surfaces) of the structure. Materials suitable for this class of elements include polyester polyols for the lower melting core and polyethylene terephthalate (PET) and two component polyurethanes for the outer layers. Such reflective elements and methods of making them are described in International Patent Publication No. WO 97/28470 (Minnesota Mining and Manufacturing Co.), the descriptions of which are incorporated herein by reference.
All-ceramic retroreflective elements can be made, which can be used in pavement markings, with greatly improved resistance to wear and the effects of weathering. These retroreflective elements are free of metals and polymeric material and are comprised of an opacified ceramic core and ceramic optical elements partially embedded into the core. The opacified ceramic cores of these composite reflective optical elements will often contain a mixture of amorphous (glass) and crystalline phases. The retroreflective element may be irregular in shape or in the form of a sphere, disc, tile, etc. The diffuse reflecting ceramic core, in combination with the transparent optical elements embedded in the surface, provides a surprisingly bright retroreflective element without the grey coloration and the susceptibility to corrosion associated with metallic specular reflectors. Moreover, such composite elements are inexpensive to manufacture and install. Such reflective elements and methods of making them are described in U.S. Pat. No. 5,774,265 (Mathers et al.), the descriptions of which are incorporated herein by reference.
Many other variations of composite retroreflective elements or aggregates are known which have a polymeric core with optical elements embedded in the core surface. See, for example, U.S. Pat. Nos. 3,252,376, 3,254,563, 4,983,458, 4,072,403, 4,652,172, and 5,268,789. Other retroreflective elements can also be constructed having a ceramic core and glass optical elements with a metallic specular coating. See, for example, U.S. Pat. Nos. 3,043,196, 3,175,935, 3,556,637, 3,274,888, and 3,486,952, and EP Publication No. 0,322,671. Retroreflective elements made from ceramics other than glass typically exhibit greater resistance to weathering and to wear. Some known embodiments also contain optical elements dispersed throughout the core. The core may be irregular in shape or may be regularly shaped e.g., spheres, tetrahedrons, discs, square tiles, etc.
Whatever the type of reflective element, the elements can be flood coated (along with anti-skid particles, if desired) onto the entire surface of the composition painted on a pavement surface. However, this can be quite expensive for ceramic beads or microspheres. Alternatively, the reflective elements can be positioned only in the most efficient optical part of the surface. Control of element placement on liquid markings is important for obtaining durable high retroreflectivity. The elements can also be included within one of the components of the pavement marking composition prior to mixing and can be applied, for example, during the spray coating of the composition.
Also, it is important to understand the mechanism of embedment of elements, whether in the form of coated pellets or beads. Surface treatments can be used to obtain appropriate sink (not too deep and not too shallow) and to control wetting and capillation of the coating up the sides of the element. Also, the cure rheology of the composition can affect the ability to sink elements.